Image tube utilizing transmissive dynode-type target



J LEMPERT June 15, 1965 IMAGE TUBE UTILIZING TRANSMISSIVE DYNODE-TYPE TARGET Filed Jan. 19. 1962 -BOOOV Fig.4.

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3,189,781 IMAGE TUBE UTILIZING TRANSMISSPJE DYNODE-TYPE TARGET Joseph Lempert, Penn Hills, Pa, assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 19, 1%2, Ser. No. 167,362 8 Ciaims. (Cl. 315-11) This invention relates to electron discharge devices and more particularly to those having storage target electrodes.

One well known type of storage target electrode tube is the image orthicon. This tube has a target structure formed of a thin film of glass. A photocathode and electrode means is provided on one side of the target for directing photoelectrons emitted from the photocathode onto one surface of the target to provide a charge pattern thereon corresponding to a pattern of light focused upon the photocathode of the tube. An electron beam of low velocity is used to scan the opposite surface of the target to deposit electrons on each elemental area of the target in proportion to the charge pattern established on the opposite side. The remaining electrons in the scanning or reading electron beam are returned to a collector electrode to provide the output signal of the image orthicon.

In the image orthicon pick up tube, the photoelectrons in striking the target initiate secondary emission from the bombarded surface or writing surface and these secondary electrons are collected by a mesh adjacent the bombarded surface. This collector mesh is held at a positive potential with respect to the cathode of the scanning electron beam. The reading action is accomplished in the image orthicon by the landing of the low velocity electrons on the front or scan side of the target. If the target has poor capability for the transverse conduction of charges, due to high resistivity, then over a period of a number of frames, more electrons will leave the writing side of the target than reach it through the target due to the reading action. The writing side of the target in this way charges to an equilibrium potential, a potential slightly more positive than the collector mesh, in which as many electrons leave the target as land on it. No further effective transfer of pattern information can take place under these circumstances and the tube loses its capability for writing. The basic problem therefore associated with the image orthicon target is that of transverse charge neutralization within a frame time while providing a target of sufficient resistivity to limit lateral leakage of charge and to provide adequate storage.

Another type of well known pick up tube is that utilizing a target exhibiting the property of electron bombardment induced conductivity. in this device the target is formed of a thin metallic film such as aluminum spaced from a photocathode. The metallic film is electron permeable and is coated with a film of insulating material on the opposite side of the metallic film with respect to the photocathode. The insulating material is of the type which exhibits the properties of electron bombardment induced conductivity such as arsenic trisulfide. A low ve locity scanning reading electron beam maintains the exposed surface of the insulating coating as a fixed potential different from the potential established on the metallic film or back plate. The high velocity electrons from the photocathode penetrate the target and induce conductivity through the target between the surface scanned by the reading electron beam and the metallic film, to establish, n the scanned surface, a charge pattern corresponding to the light pattern focused on the photocathode. The output signal may be derived from the target in a simiiar manner as that described with respect to the image orthicon in which the low velocity scan beam is utilized and the output signal is derived from the electrons returned to a collector electrode. The electron bombardment induced conductivity type of tube relies on the selection of specific materials to obtain suitable carriers, that is holes or electrons, and accordingly charge the surface in accordance with the field established across the insulating material. it is also possible to derive a signal from the conductive backplate on the insulating target member due to simple capacitive coupling when the charge is restored on the scan side of the target member. This is similar to the vidicon pick up tube type of read operation. Here again the operation of the target depends on conductivity of carriers through the target member. In this type of operation one is faced with the problem of the reduction in resolution due to lateral conduction between elements during that time when the target is bombarded with high energy electrons so as to induce conductivity therein.

In the copending application of R. J. Scheeberger entitled Electron Discharge Device, Serial No. 14,394, filed March 11, 1960, and assigned to the same assignee, there is disclosed a transmissive type dynode structure in which electrons bombard one side of a continuous target and in response thereto secondary electrons are emitted from the other side. This phenomenon is known as transmissive secondary emission. The sensitivity of these materials vary under bombardment and may provide an amplification of five to one hundred.

in particular, this invention is directed to a target utilizing transmissive secondary emission materials. The storage target produced is independent of the resistivity of the target material, since the writing action takes place on the scan side of the target. Thus, during Writing, the potential of scan side goes more positive due to secondary emission and during reading the potential of this surface is re turned back to its equilibrium potential which is slightly negative with respect to the scanning gun cathode. By utilization of this type of target, the reading and writing action on the target take place on the same surface namely the scan or read side of the target. There is obviously no need for charge neutralization through the bulk of material and it is therefore possible to use very high resistivity bulk materials.

Accordingly, it is an object of my invention to provide an improved storage target.

It is another object to provide an improved storage electrode adapted for improved image resolution.

It is a further object of my invention to provide an improved pick up tube of high sensitivity.

It is a further object of my invention to provide an improved storage target utilizing transmission secondary emission properties.

These and other objects are etfected by my invention and will be apparent from the following description taken in accordance with the accompanying drawing, throughout which like reference characters indicate like parts, and in which:

FIGURE 1 is an elevational view in section, schematically representing a pick up tube in accordance with the teachings of this invention;

FIG. 2 is an enlarged elevational view in section, illustrating the electrode assembly in FIG. 1;

FIG. 3 is an elevational view in section of a modified electrode assembly that may be embodied in the tube shown in FIG. 1; and

FIG. 4 is an elevational view in section of a modified electrode assembly that may be embodied in the tube of FIG. 1.

Referring in detail to FIG. 1, there is illustrated a pick up tube comprising a glass envelope 10. At one end of the envelope 1%) is a light transmissive face plate 12 of a suitable material such as glass having a coating 14 of V urn hydroxide.

a photoemissive material such as cesium antimony or BrAgCsU provided on theinner surface thereof. An elec- V tron gun 20 is provided at the opposite end of the envelope onto the target member 30. Positioned between the tar get member 30 and the'photocathode 14 is a grid member 40 of a conductive material such as nickel which is located at a distance of about .001 to .125 inch from the surface of the target member 30.

The target member 30 is comprised of a support ring 32 of a suitable material such as nickel having a suitable insulating support film 34 such as A1 attached to a metal ring 32 and a coating 36 of a suitable insulating material that exhibits the property of transmissive secondary emission provided on the surface of the insulating film 34 facing the electron gun 20. The coating 36 may be of a suitable alkaline or alkaline earth metal compound such as KCl, MgF or MgO. A conductive screen or mesh member .2 of a material such as nickel is also provided adjacent the transmissive secondary emissive coating 36 and between the target member 30 and the electron gun 20. This serves as a collector for secondaries from the target. The mesh 42 helps in maintaininga uniform electric field between the collector and target. In addition, a conductive coating 44 is provided on the inner wall of the envelope in the space between the electron gun 20 and the target 30 for providing a suitable electrostatic field. A mesh 46 may also be provided between the screen 42 and the gun 20 for providing a uniform decelerating field between the focusing electrode, and the collector, or between the focusing grid and target if a separate collector is not used. The use of a separate mesh allows separate adjustment of electron beam focus and the voltage of the.

collector electrode. The electron gun 20 is of any suitable type for producing a low velocity pencil-like electron beam to be scanned over the surface of the target electrode. The electron gun 20 may consist of a cathode 22, a control grid 24 and an accelerating grid 26. The gun electrodes 22, 24 and 26 along with the coating 44 provide a focused electron beam which is directed onto the target 30. Deflection means illustrated as a coil 50 is provided around the electron gun 20 for deflection of the electron beam for producing a line and frame scansion over the surface of the target 30. It is also necessary to provide a magnetic coil 52 around the envelope 10 to provide additional focusing of the electron beam onto the target 30 as well as for focusing the electrons from the photo cathode 14 onto the target 30.

A specific example of a suitable storage electrode 30 in accordance with the present invention and a method of forming such a structure will now be described. The support layer 34 is formed from a sheet of aluminum foil of a suitable thickness secured to a support ring of a suitable material such as Inconel. The thickness of the aluminum support layer should be about 60,000 angstroms in thickness for an electrode diameter of about one inch. The aluminum foil layer is anodized to the desired thickness to provide a coating of aluminum oxide of about 200 angstroms in thickness on both surfaces of the aluminum sup.- port. One aluminum oxide coating may then be removed by treatment with a suitable caustic reagent such as sodi- The aluminum layer may then be removed by a suitable acid solution such as hydrochloric to leave a thin membrane of aluminum oxide of about 200 angstroms in thickness. The membrane 34 may then be mounted on the ring 32. The ring 32 may be of nickel, glass or stainless steel. A more thorough description of this process is found in US. Patent 2,898,499 by E. J. Sternglass et 211., issued August 4, 1959. The secondary emissive material is then evaporated onto the A1 0 support layer 34 to form the layer 3-5.

One suitable method of depositing the layer 36 is to place the support layer 34 in an atmosphere of approximately one millimeter mercury of argon gas. A quantity of 16 milligrams of barium fluoride in solid or chunk form is then evaporated onto the A1 0 layer 34. The material is placed at a distance of approximately three inches below the storage support member 34. The barium fluoride is evaporated to completion and the area density of the evaporated barium fluoride layer on the aluminum oxide layer 34 is approximately 87 micrograms per square centimeter. Such a layer has a thickness of approximately 20 microns. Therefore, it is seen that while barium fluoride has a bulk density of about 4.838 grams per cubic centimeter, the porous deposit has a density of the order of about 0.04 gram per cubic centimeter.

Briefly, the operation of the tube is as follows:

The values of the potentials applied to the electrodes as illustrated in FIG. 1 are examples of operating voltages which may be used and are not meant to be limiting. The electron beam generated by the electron gun 20 is scanned over the target surface 36. The retarding field between the target film 30 and a deaccelerating electrode 46 reduces the velocity of the electron beam 38 so that it approaches the target surface at a low potential. Electrons are depositedon the target surface 36 and that surface seeks an equilibrium potential which is substantially equal to that of the cathode 22 of the electron gun 20.

An optical image is focused upon the photocathode 14 and photoelectrons are emitted from each portion of the photocathode 14 in proportion to the amount of light directed thereon. The photoelectrons emitted by the photocathode 14 are focused by the fields produced by the electrodes 16 and 18 and the coil 52 onto the surface of the target 34. The photoelectrons are accelerated to a sufficiently high velocity of about 4,000 to 8,000 volts so that they penetrate through the insulating layer 34 and into the transmissive secondary emissive layer 36. The acceleration voltage should be adjusted such that substantially all the primary electrons from the photocathode 14 almost penetrate the entire storage electrode 30 but do not pass on through the structure. The primary electrons bombarding the storage electrode 30 will cause a secondary electron yield from the surface of the layer 36 facing the electron gun 20 greater in number than the number of primary electrons striking the surface of layer 34. The secondary electrons will be collected by the collector electrode 42 adjacent the secondary emissive layer 36 so as to Charge the surface of layer 36 in a positive direction with regard to the equilibrium potential. The positive charge pattern thus generated on the surface of the secondary emissive layer 36 will correspond to the light image directed onto the photocathode.

The electron beam from gun 20 upon scanning the surface of layer 36 in these positive charged areas will deposit electrons to drive those areas back to equilibrium or gun cathode potential. The remainder of the beam is reflected by the discharged areas of the target and forms a portion of a return beam. An output signal may be derived from the mesh 40 or mesh 42 due to capacity coupling. It is also possible to utilize the return electron beam to derive an output from the tube. This latter method of deriving an output from such a scanning operation is utilized in the image orthicon type tube and is substantially conventional in the art.

In FIG. 3 there is illustrated a modification of the storage assembly as shown in FIG. 2 in that a conductive backing 54 is provided on the opposite surface of the insulating support member 34 with respect to the transmissive secondary emissive coating 36. This structure may be fabricated in a similar manner as described with respect to FIG. 2 with the addition of evaporating the aluminum layer 54 onto the support 34. The thickness of the aluminum layer 54 should be about 500 angstroms. The operation of this device is similar to that described with respect to FIGS. 1 and 2 with the exception that the output signal voltage is derived from the conductive backing 54 in FIG. 3. When the conductive coating 54 is used, the grid 40 is not necessary.

A modified target structure is shown in FIG. 4 and is similar to that shown in FIG. 2. The transmissive secondary emissive film 36 is utilized without any need for the insulating support layer 34. The operation of this device is similar with that given with respect to FIGS. 1 and 2.

It is obvious that the device illustrated in FIGS. 1 and 2 will provide a low capacity type target in comparison with that shown in FIG. 3. The structure illustrated in FIG. 4 also obtains the advantages of a low capacitance type electrode structure similarly with regard to FIGS. 1 and 2 in that the use of the vacuum dielectric between the transmissive secondary emissive el ctron layer and the conductive mesh which is spaced one mil to one-eighth inch distance from the target member.

While there have been shown and described what are presently considered to be the preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

I claim as my invention:

1. An electron discharge device comprising a target electrode including a film of insulating material having the property of emitting secondary electrons from a first surface in response to electron bombardment of a second surface, said first and second surfaces being exposed over their active areas, means for directing a writing electron beam having electrons of a predetermined energy at said second surface of said insulating film to establish at said first surface a positive charge pattern due to emission of secondary electrons from said fir surface from an equilibrium charge corresponding to said writing beam, means for directing electrons below said predetermined energy at said first surface of said film to restore said first surface to said equilibrium charge.

2. An electron discharge device comprising a target electrode including a film of insulating material exhibiting the property of emission of secondary electrons from a first surface thereof in response to bombardment by electrons of a predetermined energy on a second surface of said film, means for directing a first electron beam be low a predetermined energy at said first surface of said insulating film to establish said first surface of said film at an equilibrium potential, means for directing a second electron beam of said predetermined energy at said second surface of said film to penetrate into said insulating film at said second surface so that substantially all the electrons in said second electron beam almost penetrate through said insulating film but do not pass through said insulating film to excite secondary electrons from said first surface of said film which was at equilibrium potential to charge said first surface positively with respect to said equilibrium potential and means adjacent said first surface to collect said secondary electrons emitted therefrom to establish a charge pattern on the first surface corresponding to the electrons directed onto said second surface of the film, and means for deriving an output signal corresponding to said charge pattern by scanning said first electron beam over said first surface while simultaneously restoring said first surface to equilibrium potential.

3. An electron discharge device comprising a target electrode including a sheet of insulating material having the property of emitting secondary electrons from one surface in response to electron bombardment of the opposite surface, means for directing electrons of a first energy at a first surface of said insulating sheet to establish said first surface at an equilibrium charge potential and means for directing electrons of a second energy at the second surface of said sheet to penetrate into said insulating sheet at said second surface so that substantially all the electrons of said second energy almost penetrate through said insulating sheet but do not pass through said insulating sheet to establish a positive charge at said first surface due to emission of secondary electrons from said first surface.

4. An electron discharge device comprising a target electrode including a film of insulating material exhibiting the property of emission of secondary electrons from a first surface thereof in response to electrons entering a second surface of said film at a first energy, means for directing electrons of a second energy at said first surface of said insulating film to establish the surface of said film at an equilibrium potential, means for directing electrons of said first energy at said second surface of said film to penetrate into said insulating film at said second surface so that substantially all the electrons in said second electron beam almost penetrate through said insulating film but do not pass through said insulating film to excite secondary electrons from said first surface of said film which was at equilibrium potential so as to charge said first surface positively with respect to said equilibrium potential and means adjacent said equilibrium surface to collect said secondary electrons.

5. An electron discharge device comprising a target electrode including an electrically conductive sheet, a film of insulating material provided on said conductive sheet and having the property of emitting secondary electrons from the exposed surface in response to electron bombardment of the conductive sheet, means for directing a scanning electron beam of electrons having a first energy at said exposed surface of said insulating film to establish said exposed surface at an equilibrium potential, means for directing an electron image of electrons of a second energy at said conductive sheet to pass through said conductive sheet and penetrate into said insulating film so that substantially all of the electrons of said electron image almost penetrate through said insulating film but do not pass through said insulating film to establish a positive charge image at said exposed surface corresponding to said electron image, said positive charge image due to emission of electrons from said exposed surface in response to excitation of said film by electrons within said electron image.

6. An electron discharge device comprising an evacu ated envelope, an electron gun to develop an electron beam, 21 target comprising a sheet of material exhibiting the property of emission of secondary electrons from one surface in response to electron bombardment of the other surface, both surfaces of said sheet being exposed over their entire area within said envelope, a screen member closely adjacent to an exposed surface of said target and substantially parallel thereto, means to scan said electron beam over one of said exposed surfaces of said sheet and means including a photocathode to generate and direct a photoelectron image onto the other surface of said target to develop a potential image on the surface of said sheet facing said electron gun.

7. An electron discharge device comprising a tube having an evacuated envelope, means within said envelope adapted to generate an electron beam, a target member comprising a sheet of insulating material and having the property of emitting secondary electrons from an exposed surface in response to electron bombardment of the other surface, a support member positioned on one side of said sheet of insulating material, said electron beam generating means positioned so as to scan the exposed surface of said insulating sheet, a second electron beam source for forming an electrical charge image on the exposed surface of said insulating layer and positioned on the side of said target facing said support layer and a conductive mesh electrode positioned on the support side of said target and in substantial capacitive relationship therewith.

8. An electron discharge device comprising an evacuated envelope, means for generating a first electron beam of small spot size, a target comprising a layer of high resistive material exhibiting the property of generating secondary electrons from an exposed surface in response to bombardment of the other surface, means for directing said first electron beam onto said exposed surface of said layer, a first electron permeable electrode co-planar with and separated from the exposed surface of said layer for collecting the secondary electrons emitted from the exposed surface of said layer, a second electron permeable electrode co-planar with and separated from the opposite References Cited by the Examiner UNITED STATES PATENTS 3/51 Townes 313-65 8/59 Sternglass 313-65 DAVID G. REDINBAUGH, Primary Examiner.

surface of said layer and having substantial capacitive 15 ARTHUR GAUSS, Examiner. 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING A TARGET ELECTRODE INCLUDING A FILM OF INSULATING MATERIAL HAVING THE PROPERTY OF EMITTING SECONDARY ELECTRONS FROM A FIRST SURFACE IN RESPONSE TO ELECTRON BOMBARDMENT OF A SECOND SURFACE, SAID FIRST AND SECOND SURFACES BEING EXPOSED OVER THEIR ACTIVE AREAS, MEANS FOR DIRECTING A WRITING ELECTRON BEAM HAVING ELECTRONS OF A PREDETERMINED ENERGY AT SAID SECOND SURFACE OF SAID INSULATING FILM TO ESTABLISH AT SAID FIRST SURFACE A POSITIVE CHARGE PATTERN DURE TO EMISSION OF SECONDARY ELECTRONS FROM SAID FIRST SURFACE FROM AN EQUILIBRIUM CHARGE CORRESPONDING TO SAID WRITING BEAM, MEANS FOR DIRECTING ELECTRONS BELOW SAID PREDETERMINED ENERGY AT SAID FIRST SURFACE OF SAID FILM TO RESTORE SAID FIRST SURFACE TO SAID EQUILIBRIUM CHARGE 